1. Technical Field
The present invention relates generally to a microwave directional coupler, and specifically to such a coupler having interdigital coupling line elements disposed on opposing sides of a dielectric substrate which determine the even and odd mode impedances of the coupler independent of the parent circuit board on which the coupler is mounted.
2. Description of the Prior Art
A microwave directional coupler is a four port microwave device used as a power divider or combiner. The four ports are designated Port 1, Port 2, Port 3, and Port 4. When a signal is input to Port 1, it is coupled into Ports 2 and 3 but not into Port 4. A signal input to Port 4 is similarly coupled into Ports 2 and 3 but not into Port 1. Because there is no coupling between Ports 1 and 4, these ports are known as uncoupled or isolated ports relative to each other. Signals may also be input, or result from reflections, in Ports 2 and 3. A signal input to Port 2 is coupled to Ports 1 and 4 but not to Port 3, while a signal input to Port 3 is coupled to Ports 1 and 4 but not to Port 2. Thus, Ports 2 and 3 are isolated ports relative to each other.
A typical application for a directional coupler is in a radar system for monitoring transmitter power. In a radar system, the radar transmitter is connected to Port 1 of the directional coupler. The antenna is then connected to Port 2, a microwave power detector to Port 3, and Port 4 is terminated in a matched load. With this configuration, any impedance mismatch of the antenna will not result in the reflected power propagating into Port 2 being measured in Port 3 as power output of the transmitter. This is true because Ports 2 and 3 are isolated ports and a signal input to Port 2 is not coupled to Port 3.
A microstrip directional coupler is disclosed in U.S. Pat. No. 4,823,097 to Konishi et al. The coupler comprises a first dielectric board which stands erect on the upper surface of a parent dielectric board. A coupling line part is disposed on each side of the first dielectric board. Lead lines are disposed on the upper surface of the parent dielectric board and are connected to both ends of each coupling line part. A ground plane is formed by a conductive layer disposed on the lower surface of the parent dielectric board. This physical construction results in the even and odd mode impedances of the coupler being determined by both coupling between the coupling line parts and by coupling between each coupling line part and the ground plane. Because the impedances are determined in part by coupling to the ground plane, the composition and physical dimensions of the parent dielectric board affect the impedances. Therefore, the design of the coupler requires knowledge and consideration of the type of parent dielectric board on which the first dielectric board will be mounted.
Another embodiment of a microstrip directional coupler is disclosed in U.S. Pat. No. 5,373,266 to Lenzing et al. This coupler is comprised of a pair of microstrip coupling elements having remote edges which are straight and adjacent edges which follow curved paths having reversals in curvature such as sinusoidal or half circle patterns. The coupling elements are disposed on one surface of a dielectric substrate. A ground plane conducting layer is disposed on the opposite surface of the dielectric substrate. In this coupler, the even mode impedance is formed by coupling between the straight remote edges of the microstrip coupling elements and the ground plane. Therefore, the composition and physical dimensions of the single dielectric substrate affect the coupler even mode impedance and thus the coupler design.
A directional coupler including comb electrodes having an elongated bus bar with a plurality of spaced teeth is disclosed in U.S. Pat. No. 4,394,630 to Kenyon et al. In one embodiment, a first comb electrode is disposed on one side of a dielectric substrate and a second comb electrode is disposed on the other side of the dielectric substrate. A generally rectangular hollow conductor encases and supports the dielectric substrate. A filling material having predetermined dielectric characteristics is used to fill the volume between the one side of the dielectric substrate and the adjacent side of the hollow rectangular conductor. In the same way, a filling material is used to fill the volume between the other side of the dielectric substrate and the adjacent side of the hollow rectangular conductor. The even and odd mode impedances of the coupler are determined by the composition and physical dimensions of both the dielectric substrate and the filling material.
A drawback common to these prior art couplers is that the conductor line patterns which form the coupling elements are required to be printed on the parent printed circuit board containing the other peripheral electronic circuitry. A design conflict arises in this situation where the directional coupler may require an expensive dielectric material to achieve the required performance while a cheaper material would suffice for the peripheral circuitry. Because the coupler and the peripheral circuitry are contained on the same circuit board, the use of an expensive material for the entire board is required to meet coupler design criteria.
A further disadvantage in prior art couplers is where coupling is produced from structures having mixed dielectrics or where the fields are contained in media having different permittivity values. The different permittivity values produce differing phase velocities of the fields which result in non-optimum coupler directivity. Further, it is known by those skilled in the art that other effects which limit the performance of directional couplers are so called "end effects", where discontinuities occur between the unbalanced feed connections and the balanced coupled lines. Additionally, in view of increasing trends toward automated component placing and reflow soldering, some coupler types do not lend themselves to automated assembly and manual assembly is required.